The present invention generally relates to semiconductor plasma etching. More particularly, the invention relates to a probe interface and method for measuring radio frequency (RF) power delivered to a plasma chamber that improves the dissipation of heat resulting from the RF power.
In the semiconductor industry, plasma etching has become an integral part of the manufacture of semiconductor circuits. Plasma etching, which uses ions accelerated by an electric field, tends to etch only horizontal exposed surfaces and therefore avoids undercutting. A typical plasma etching system will include a plasma chamber (containing the silicon wafer to be etched), a power delivery system, and a feedback system for providing closed loop control of the RF power delivered to the plasma chamber. The feedback system typically includes a probe head that generates analog signals based on the RF power flowing through a conductor and a probe analysis system that processes the analog signals into useful data such as voltage magnitude, current magnitude, and relative phase digital data.
It is important to note that the probe head is relatively small in size in comparison to the plasma chamber and the power delivery system. It is common for a probe head to include a housing containing a probe xe2x80x9cinterfacexe2x80x9d, where the probe interface is made up of the conductor, sensing boards and an electrically insulative bushing. The conductor is placed in series with the power line connected between the power delivery system and the plasma chamber. Thus, the conductor participates in transmitting the RF power to the plasma chamber. The sensing boards typically include a voltage pick-up board and a current pick-up board for generating a voltage analog signal and a current analog signal based on the RF power flowing through the conductor.
The bushing is inserted over the conductor and is electrically insulative in order to prevent electric shock to individuals touching the housing. The bushing also helps to minimize electrical losses. To serve this purpose, the conventional bushing is made of polytetraflouethylene (PTFExe2x80x94commercially known as Teflon), which has a dielectric constant of approximately 2.85. Thus, the relatively low dielectric constant (i.e., high dielectric strength) enables the bushing to be an electrical insulator between the conductor and the sensing boards.
While the above-described probe interface has been effective in the past, there remains considerable room for improvement. For example, as the size of silicon wafers increases (as well as the frequency of the RF power), the amount of RF power needed also increases. The electrical current associated with the power generates heat in the conductor. On the other hand, the semiconductor industry also requires small probe heads. Thus, the conductor cannot be increased in size to account for the increase in current. The result is often a greater occurrence of thermal instability and a limited power handling capacity. In fact, Teflon has a relatively low thermal conductivity of approximately 0.41 watts per meter per degree Kelvin and causes the bushing to act as a thermal insulator to the conductor. The thermal characteristics of Teflon are discussed in greater detail in xe2x80x9cThermal Conductivity of PTFE and PTFE Compositesxe2x80x9d, D. M. Price et al., Proceedings of the 28th Conference of the North American Thermal Analysis Society, Oct. 4-6, 2000, pp. 579-584. It is therefore desirable to provide a probe interface bushing that can more effectively dissipate heat resulting from RF power.
The above and other objectives are provided by a probe interface for an RF probe head in accordance with the present invention. The interface includes a conductor for transmitting RF power to a plasma chamber, and a sensing board for generating an analog signal based on the RF power. An electrically insulative bushing is disposed between the conductor and the sensing board, where the bushing is thermally conductive such that the bushing dissipates heat resulting from the RF power. By enabling the bushing to be thermally conductive, thermal instability is reduced and the overall RF power handling capacity increases.
Further in accordance with the present invention, an electrically insulative bushing for an RF probe head interface is provided. The bushing includes a thermally conductive material, and a binder material dispersed throughout the conductive material. The materials combine to provide a predetermined level of thermal conductivity. In the preferred embodiment, the conductive material includes a ceramic material such a boron nitride.
In another aspect of the invention, a method for measuring RF power delivered to a plasma chamber is provided. The method includes the step of transmitting the RF power to the plasma chamber via a conductor. A sensing board is used to generate an analog signal based on the RF power. The method further provides for disposing an electrically insulative bushing between the conductor and the sensing board, where the bushing is thermally conductive such that the bushing dissipates heat resulting from the RF power.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.